drm: trivial documentation fix to drm_for_each_connector_iter

While reading drm_for_each_connector_iter, I noticed a mention to
drm_connector_begin which doesn't exist. It should be
drm_connector_get.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421003819.17685-1-krisman@collabora.co.uk